Liquid crystal displays (LCDs) are widely used in information displays. Due to the intrinsic optical anisotropy of liquid crystal materials, the incident light “sees” different effective birefringence when viewed from different directions. For this reason, the viewing angle of conventional LCDs is not as wide as the viewing angle for self-luminescence displays, such as the cathode-ray tube (CRT), organic light-emitting diode and the plasma display panel. In an effort to widen the viewing angle, several display modes have been disclosed using a lateral electric field to activate the liquid crystal (LC) molecules. In-plane switching (IPS) mode is disclosed in M. Oh-e, et al., “Principles and characteristics of electro-optical behavior with in-plane switching mode”, Asia Display, 95, pp. 577-580 (1995) and U.S. Pat. No. 5,600,464, issued to Oh-e in 1997, and fringe field switching mode (FFS) is disclosed in S. H. Lee et al., “Electro-optic characteristics and switching principles of a nematic liquid crystal cell controlled by fringe-field switching”, Appl. Phys. Lett., Vol. 73, pp 2881-2883 (1998) and U.S. Pat. No. 5,886,762, issued to Lee in 1999.
In both IPS and FFS modes, the LC molecules at voltage-off state are basically homogeneously aligned on glass or plastic substrates that are coated with a thin indium-tin-oxide (ITO) layer and then overcoated with a polyimide alignment layer. The surfaces of polyimide layers are rubbed in parallel or anti-parallel directions to create a homogeneous alignment. The display panel is sandwiched between two crossed polarizers, and the long axis of LC molecules is either parallel or perpendicular to the transmission direction of their adjacent polarizers at off-state. At on-state, the lateral electric field generated from the comb-shaped electrodes cause the molecules to twist within the plane parallel to the supporting substrates. Therefore, from the opposite direction of the display panel, the incident light experiences almost the same birefringence and a relatively wide and symmetric angle is achieved.
However, the two orthogonally crossed polarizers are no longer perpendicular to each other when viewed from the oblique off-axis direction, especially from the bisector of the crossed polarizers. FIGS. 1A and 1B are schematic diagrams of the crossed polarizers under normal view and under an oblique view from polar angle θ at the bisector of the crossed polarizers, respectively, where the solid line 11 represents the absorption direction of the top polarizer and the dashed line 12 represents the absorption direction of the bottom polarizer. As shown in FIG. 1B, the absorption axes of these two crossed polarizers make an angle of 2 tan−1(cos θ), depending on the viewing polar angle θ. As the viewing polar angle θ increases, the angle between the two crossed polarizers deviates further from 90°. As a result, light leakage increases as the polar angle increases. For example, FIG. 2 illustrates the typical view of crossed polarizers, wherein the numbers in the contour polar plot represent the transmittance of the incidence light and the wavelength is approximately 550 nm. At azimuthal angles of 45°, 135°, 225° and 315° in FIG. 2, the light leakage is prominent when the polar angle increases approximately 70°. In FIGS. 3A through 3C, the viewing angle characteristics for a conventional IPS mode LCD are illustrated wherein the LC rubbing direction is along the horizontal (0°) axis and the pretilt angle is 1°. The figures illustrate that due to the light leakage at the voltage-off state, the 10:1 contrast ratio contour is limited to a 70° polar angle at azimuthal angles of 45°, 135°, 225° and 315°.
Compensation methods have been disclosed for solving the light leakage problem associated with crossed polarizers. In Chen et al., “Optimum film compensation modes for TN and VA LCDs”, SID 1998 Digest, pp 315-318 (1998) and J. E. Anderson and P. J. Bos, “Methods and concerns of compensating in-plane switching liquid crystal displays”, Jpn. J. Appl. Phys., Vol. 39, pp 6388-6392 (2000), a method is disclosed for using a positive birefringence C-film (nx=ny<nz) plus a positive birefringence A-film (nx>ny=nz), where the z-axis is along the film surface normal direction, i.e. the film thickness direction and x axis is parallel to the optical axis direction. An alternative method using a single biaxial film (nx>ny>nz) to compensate for the light leakage of crossed polarizers is disclosed in Y. Saitoh et al., “Optimum film compensation of viewing angle of contract in in-plane-switching-mode liquid crystal display”, Jpn. J. Appl. Phys. Part 1, Vol. 37, pp 4822-4828 (1998). In addition, a design using two biaxial films to compensate light leakage in a large wavelength range is disclosed in T. Ishinable et al., “A wide viewing angle polarizer and a quarter-wave plate with a wide wavelength range for extremely high quality LCDs”, IDW'01, pp 485-488 (2001) and T. Ishinable et al., “A wide viewing angle polarizer with a large wavelength range”, Jpn. Appl. Phys. Part 1, Vol. 41, pp. 4553-4558 (2002). However, the cost associated with a C-film and a biaxial film is much higher than the cost of A-film. Additionally, use of a combination of a C-film and an A-film or a single biaxial film does not achieve the desired symmetric viewing angle.
The present invention advances the art by providing a method and apparatus using a positive uniaxial A-film and a negative uniaxial A-film to compensate the dark state light leakage of the liquid crystal display, in which the liquid crystal molecules are homogeneously aligned at inactive state when no voltage is applied to liquid crystal layer and are driven by a substantially lateral electric field. After compensation, the dark state light leakage is greatly decreased and the contrast ratio at oblique viewing polar angle is greatly enhanced, as a result, more than 100:1 contras ratio is achieved in all viewing angles. During the analysis process of dark state light leakage, the Poincaré sphere presentation is used to illuminate the polarization state change in liquid crystal panel.